New treatment modalities are desperately needed for patients with metastatic solid tumors. Ideally, these strategies would focus cytotoxic effects on the tumor cells and spare healthy normal tissues. One such approach known as Radioimmunotherapy (RAIT) entails employing anti-tumor antibodies to deliver cytotoxic agents such as radioisotopes. While RAIT has been effective in the treatment of hematological malignancies, its utility against solid tumors is limited by physiological factors that prevent the tumor accumulation of therapeutic quantities of radioactivity. We hypothesize that this limitation could be overcome by increasing the specific activity of the immunoconjugates through the use of radioactive nanoparticles. We have developed rapid "one pot" methodology to produce small (<20 kDa) functionalized radioactive nanoparticles containing scores of radioactive rhenium atoms. In the current application, we will conjugate these nanoparticles to engineered anti-HER2 single-chain Fv molecules and evaluate their in vitro and in vivo targeting properties. In the first specific aim, we will produce, characterize and optimize the molecules in in vitro assays. In the second specific aim, we will evaluate their performance focusing on determining their pharmacokinetics, biodistribution and therapeutic efficacy in two in vivo models;immunodeficient mice bearing HER2 + human tumor xenografts and transgenic mice expressing human HER2 on normal tissues. The latter studies will provide critical information on the impact of background HER2 expression on the targeting efficiency, normal tissue toxicity and therapeutic efficacy of these novel agents. At the conclusion of the proposed studies we will have an accurate assessment of the ability of these radioactive nanoparticle immunoconjugates to target and treat tumors in highly relevant animal models and will have acquired an indication regarding potential toxicities that could be encountered in the clinical setting. It is our intention to rapidly translate promising conjugates into clinical trials in subsequent proposals. Public Health Statement The proposed research is focused on developing a new targeted treatment for solid tumors. If the preclinical studies described here demonstrate that targeted radioactive nanoparticles are associated with a favorable therapeutic efficacy with minimal normal tissue toxicity, we will proceed as rapidly as possible to the clinical setting.